Diaphragm pumps using a diaphragm made of synthetic resin thin film are being used in various industrial fields including the chemical industry, the pharmaceutical industry, the semiconductor industry and the printing industry because of the advantages they provide including that the liquid can be transferred without being damaged, that it is not necessary to use an anti-leakage seal member and that it can be arranged so that liquid does not contact any metal.
However, such diaphragm pumps normally generate pulsation because liquid is taken in and discharged by reciprocating the diaphragm.
Arrangements of combining a pair of diaphragm pumps and using them complementarily so as not to generate any pulsation at the liquid discharge side are proposed for the purpose of suppressing the pulsation of a diaphragm pump (see, for instance, Reference 1: Japanese Patent Laid-Open Publication No. 2003-042069).
In addition, arrangements of sequentially closing three chambers with diaphragms, which functions as a pump without providing a check valve, has been also proposed (see, for instance, Reference 2: specification of U.S. Pat. No. 5,593,290).
However, such combined diaphragm pumps disclosed in Reference 1 are provided with a check valve for preventing liquid from flowing backward. In other words, they are accompanied by a problem that they cannot allow liquid to flow back.
In the pump disclosed in Reference 2, since the diaphragm is deformed by a liquid, it is difficult to speed up a drive operation, and since chambers of plural systems are provided in parallel, it is difficult to reduce size and weight.